This invention relates to syringe pumps. More specifically, this invention describes a high-pressure micro-volume syringe pump particularly suited to for analytical separations.
An important trend in modem analytical chemistry has been the move towards separation techniques capable of accommodating small sample volumes, i.e., sample volumes in the range of 1 to 10 .mu.l. This trend is particularly strong in the area of analytical biotechnology where samples are frequently derived from scarce natural isolates or from valuable recombinant products. Typical analytical biotechnology applications include chromatographic separations used as part of protein sequencing operations, amino acid analysis, protein/peptide mapping, quality control of pharmaceutical products, and the like.
To avoid dilution of the sample and thereby maintain the delectability of the separated components, the scale of the separation columns, e.g., chromatographic columns, has been reduced to match the scale of the samples, such micro-scale columns having internal diameters as small as 50 .mu.m. An added benefit of scaling down the separation equipment is the reduced volume of working fluid required, e.g., chromatographic solvents and/or eluants, leading to reduced costs for acquiring and disposing of such materials, particularly in the case of exotic and/or highly toxic materials.
Micro-scale separations place a particular burden on the pumps used to deliver the working fluid to the separation column. The performance characteristics of typical HPLC pumps is not adequate to satisfy the exacting demands of such micro-column separations-where an error of .+-.1 .mu.l might be undetectable in a HPLC application running at a flow rate of 2 ml/min, that same error could lead to unacceptably large errors in a micro-column application running at a flow rate of less than 10 .mu.l/min.
Syringe pumps are well suited to the demands of micro-column chromatographic separations. Syringe pumps have several advantages over reciprocating pumps when used for micro-scale analytical separations, e.g., liquid chromatography, super critical fluid chromatography, and the like, including (i) essentially pulse-free fluid flow and (ii) highly reproducible and accurate volumetric fluid delivery.
However, currently available syringe pumps have a number of important shortcomings. In particular, existing syringe pumps are not able to deliver low solvent flow rates at high pressure with the requisite accuracy and precision desirable for analytical separations. Furthermore, existing syringe pumps transmit a high level of mechanical vibrations to the working fluid, thereby interfering with detection of the separated sample components. Another drawback of existing syringe pumps is that wear on moving sealing surfaces is such that parts including such sealing surfaces frequently wear out, leading to poor run-to-run reproducibility and necessitating frequent pump disassembly and replacement of the worn parts.
When used in a multiple-pump gradient mode, because of the shortcomings noted above, existing syringe pumps are unable to produce reproducible gradients, particularly at very low solvent flow rates and at high pressure. To achieve low flow rates in a multiple-pump gradient mode, existing syringe pumps require the use of a solvent splitter which serves to direct a portion of the outflow from the pump to a waste stream rather to the separation column, e.g., Moritz et at., Journal of Chromatography 599:119-130 (1992). Such splitting techniques introduce large errors in the solvent delivery profile due to changes in the solvent density and viscosity as the composition of the solvent is changed throughout the gradient. In addition, existing systems require mixers which have relatively large internal volumes, introduce significant noise into the flow stream, and release particulates into the flow stream as a result of wear of the moving parts.